Dimming technologies are widely used in lighting systems such as incandescent lamp lighting, energy-saving lamp lighting and LED lighting. FIG. 1 schematically illustrates a conventional lighting system 10 including nonlinear light-emitting devices such as LED. The lighting system 10 comprises a power supply 101, a dimmer 102, a bridge rectifier 103, a light-emitting device 106 and a driving circuit 100. In some applications, the power supply 101 could provide the residential electricity to the dimmer 102. The dimmer 102 has an input terminal configured to receive an input signal S1 and an output terminal configured to provide a dimmed signal S2. Normally, a three terminal bidirectional thyristor switch tube (TRIAC) is served as the dimmer 102. The brightness of the light-emitting device 106 would be changed by adjusting the ON time of the TRIAC. The bridge rectifier 103 has an input terminal configured to receive the dimmed signal S2 and an output terminal configured to provide a half wave signal S3. The driving circuit 100 comprises a detecting circuit 104 and a control circuit 105. The detecting circuit 104 detects the ON time or the OFF time of the dimmer 102 based on the half wave signal S3, and provides a detecting signal S4 at the output terminal. Based on the ON time or the duty cycle of the detecting signal S4, the control circuit 105 adjusts voltage or average current of the light-emitting device 106, so as to adjust the brightness.
In an ideal case, the input signal S1 and the dimmed signal S2 would be exactly the same if the TRIAC is fully turned on. However, in most cases, the duty cycle of the TRIAC may only be 85% or lower even if the knob (dimming button) of the TRIAC is turned to its maximum conduction status, such as shown in FIG. 2. Furthermore, different dimmers have different maximum duty cycles since they have different conduction characteristics, which leads to different maximum output power for different lighting systems. This may cause light-emitting devices in the same region to have different brightness. Designing lighting systems that have uniform maximum brightness for different dimmers becomes a challenge.